A&P The Skeletal System: Bone Tissue

Nutrient Artery

*Near the center of the diaphysis, a large nutrient artery passes through a hole in the compact bone called the nutrient foramen. *In the medullary cavity, it divides into proximal and distal branches which supply both the inner part of compact tissue of the diaphysis and the spongy bone tissue and red bone marrow as far as the epiphyseal plate *Some bones have more than one nutrient artery

Intramembranous Ossification Formation of Trabeculae

-As the extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue -The connective tissue associated with blood vessels in the trabeculae turn into red bone marrow

Spongy Bone

*Also referred to as trabecular or cancellous bone tissue *Does not contain osteons *Always located in the interior of the bone protected by a covering of compact bone

Intramembranous Ossification Development of the Ossification Center

-At the site where the bone will develop, specific chemical messages cause mesenchymal cells to cluster -These cells differentiate into osteoprogenitor cells, then into osteoblasts -The cluster site is called the ossification center -Osteoblasts secrete extracellular matrix until they are surrounded by it

Intramembranous Ossification Development of the periosteum

-The mesenchyme condenses at the periphery of the bone and develops into periosteum -Eventually, compact bone replaces the surface layers of spongy bone

Intramembranous Ossification Calcification

-The secretion of the extracellular matrix stops -The now osteocytes lie in lacunae and extend their cytoplasmic processes into canaliculi that radiate in all directions -Calcium and other mineral salts are deposited and the extracellular matrix calcifies

The initial formation of bone in an embryo or fetus follows one of two patterns

1. Intramembranous Ossification Bone forms directly within mesenchyme, which is arranged in sheet like layers that resemble membranes 2. Endochondral Ossification Bone forms within hyaline cartilage that develops from mesenchyme

Veins that carry blood away from long bones are found in three places

1. One or two nutrient veins accompany the nutrient artery and exit through the diaphysis 2. Numerous epiphyseal veins and metaphyseal veins accompany their respective arteries and exit through the epiphyses and metaphyses 3. Many small periosteal veins accompany their respective arteries and exit through the periosteum

Bone formation occurs in 4 principal situations

1. The initial formation of bones in an embryo and fetus 2. The growth of bones during infancy, childhood and adolesense until their adult size is reached 3. The remodeling of bone (the replacement of old bone by new bone tissue throughout life) 4. The repair of fractures (breaks in bones) throughout life

Functions of Bone and the Skeletal System

1.Support 2.Movement 3.Mineral Homeostasis 4.Blood Cell Production 5.Triglyceride Storage

Osteoporosis

A condition of porous bone Bone mass becomes so depleted that bones easily fracture Can occur If levels of a parathyroid hormones remain elevated fro a long period of time If osteoclast activity increases, while osteoblasts activity decreases Following menopause in women, due to decrease in estrogen levels from the ovaries

Greenstick Fracture

A partial fracture in which one side of the bone is broken and the other side bends Occurs only in children whose bones are not fully ossified and contain more organic material than inorganic material

Stress Fracture

A series of microscopic fractures in bone that forms without evidence of injury to other tissues Result from repeated strenuous activity

Reactive Phase

An early inflammatory phase Blood vessels crossing the fracture line are broken A clot forms around the site of a fracture Called a fracture hematoma Forms 6-8 hrs after injury Nearby bone cells die Swelling and inflammation occur in response Phagocytes and and osteoclasts remove the dead or damaged tissue in and around the fracture hematoma Stage may last several weeks

Growth in Length

At the bone surface, periosteal cells differentiate into osteoblasts, which secrete bone extracellular matrix The osteoblasts become surrounded and turn into osteocytes This forms ridges on either side of a periosteal blood vessel The ridges enlarge and create a groove for the blood vessel The ridges fold together and fuse, and the groove becomes a tunnel that encloses the blood vessel The former periosteum becomes the endosteum that lines the tunnel Osteoblasts in the endosteum deposit bone extracellular matrix, forming new concentric lamellae which continue inward toward the periosteal blood vessel The tunnel fills in and a new osteon is created As an osteon is forming, osteoblasts under the periosteum deposit new circumferential lamellae, further increasing bone thickness As addition periosteal blood vessels become enclosed, the process continues

Endochondral Ossification Development of the Cartilage Model

At the site where the bone is to be formed, chemical messengers cause mesenchyme to clump together into the shape of a bone Mesenchyme turns into chondroblasts Chondroblasts secrete cartilage extracellular matrix, producing a cartilage model made of hyaline cartilage Perichondrium develops around the cartilage model

Reparative Phase: Fibrocartilaginous callus formation

Blood vessels grow into the fracture hematoma and phagocytes begin to clean up dead bone cells Fibroblasts from the periosteum invade the fracture site and produce collagen fibers Cells from the periosteum develop into chondroblasts and produce fibrocartilage in this region These events lead to the development of a fibrocartilaginous (soft) callus A mass of repair tissue consisting of collagen fibers and cartilage that bridges the broken ends of bone Formation takes aprox. 3 weeks

Remodeling of Bone

Bone remodeling is the ongoing replacement of old bone tissue with new bone tissue Involves bone resorption- the removal of minerals and collagen fibers from bone by osteoclasts Involves bone deposition- the addition of minerals and collagen fibers to bone by osteoblasts Bone remodeling is affected by exercise, a sedentary lifestyle and changes in diet

Mineral Homeostasis

Bone tissue stores several minerals including calcium and phosphorus, which contribute to bone strength Bone tissue stores 99% of body's calcium Bone releases minerals into the bloodstream to maintain mineral balance in body

Osteoblasts

Bone-building cells Synthesize and secrete collagen fibers and other organic compounds needed to build the extracellular matrix of bone tissue Initiate calcification As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes

Endochondral Ossification Growth of the Cartilage Model

Chondroblasts become Chondrocytes Interstitial Growth occurs Growth in length from within as chondrocytes multiply and more extracellular matrix is produced Appositional Growth occurs Growth in thickness due to increase of extracellular matrix on the outer surface Outer surface of bone begins to calcify. Lacunae develop

Bone Matrix

Composed of water, collagen fibers and crystallized mineral salts (calcium phosphate

Zone of Hypertrophic Cartilage

Consists of large, maturing chondrocytes arranged in columns

Bone Remodeling Phase

Dead portions of the original fragments of bone are gradually reabsorbed by osteoclasts Compact bone replaces spongy bone around the periphery of the fracture A thickened area on the surface of the bone remains as evidence of a healed fracture

Endochondral Ossification

Development of the Cartilage Model Growth of the Cartilage Model Development of the Primary Ossification Center Development of the Medullary (Marrow) Cavity Development of the Secondary Ossification Centers Formation of Articular Cartilage and the Epiphyseal (Growth) Plate

Intramembranous Ossification

Development of the Ossification Center Calcification Formation of Trabeculae Development of the periosteum

Osteoclasts

Huge cells derived from the fusion of as many as 50 monocytes (type of white blood cell) Are concentrated in the endosteum The cell releases powerful lysosomal enzymes and acids, from the ruffled border of its cell membrane, which digest the protein and mineral components of the underlying extracellular bone matrix This breakdown is classified as resorption Is part of the normal development, maintenance and repair of bone

Reparative Phase: Bony callus formation

In areas closer to the healthy bone tissue, osteoprogenitor cells develop into osteoblasts Begin to produce spongy bone trabeculae The trabeculae join living and dead portions of the origional boe fragments Fibrocartilage is converted into spongy bone The callus is then referred to as a bony (hard) callus Lasts 3-4 months

Hormones

Insulin growth factors (IGFs) are the most important for bone growth Produced by the liver and bone tissue Stimulate osteoblasts, promote cell division at the epiphyseal plate and in the periosteum, and enhance synthesis of the proteins needed to build new bone Produced in response to the secretion of human growth hormone Sex hormones are responsible for increased osteoblast activity, synthesis of bone extracellular matrix, and the sudden "growth spurt" that occurs during teenage years

Growth in Length Involves Two Major Events

Interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate Replacement of cartilage on the epiphyseal side of the epiphyseal plate with bone by endochondral ossification

Minerals

Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller amounts of magnesium, fluoride and manganese Necessary during bone remodeling

Osteocytes

Mature bone cells, the main cells in bone tissue Maintains the daily metabolism of bone through the exchange of nutrients and wastes with blood

Factors Affecting Bone Growth

Minerals Vitamins Hormones

Movement

Most skeletal muscles attach to bones- when the muscles contract they pull the bones to produce movement

Zone of Resting Cartilage

Nearest to the epiphysis Consists of small, scattered chondrocytes Cells do not function in bone growth Anchor epiphyseal plate to the epiphysis of the bone

Zone of Calcified Cartilage

Only a few cells thick and consists mostly of dead chondrocytes because the extracellular matrix surrounding them had calcified Osteoclasts dissolve the calcified cartilage and osteoblasts and capillaries from the diaphysis invade the area Osteoblasts lay down bone extracellular matrix, replacing the calcified cartilage Process called endochondral ossification The zone of calcified cartilage becomes the new diaphysis

Osteogenic cells

Or Osteoprogenitor cells Unspecialized bone stem cells derived from mesenchyme The only bone cells to undergo cell division Resulting cells are osteoblasts Found along inner portion of periosteum, in the endosteum, and in the canals within bone that contain blood vessels

Endochondral Ossification Development of the Medullary (Marrow) Cavity

Osteoclasts break down some of the newly formed spongy bone trabeculae This results in the medullary cavity in the diaphysis

Endochondral Ossification Development of the Primary Ossification Center

Primary ossification proceeds inward from the external surface of the bone A nutrient artery penetrates through perichondrium and goes through nutrient foramen Stimulates osteoprogenitor cells in the perichondrium to differentiate into osteoblasts Once the perichondrium starts to form bone, it is the known as the periosteum Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, including the growth of the primary ossification center, a region where bone tissue will replace most of cartilage Osteoblasts deposit bone extracellular matrix over the remnants of the calcified cartilage, forming spongy bone trabeculae Primary ossification spreads from the central location towards both ends of the model

Phases of repair of a bone fracture

Reactive Phase Reparative Phase: Fibrocartilaginous callus formation Reparative Phase: Bony callus formation Bone Remodeling Phase

Rickets

Result from inadequate calcification of the extracellular bone matrix, Usually caused by a Vitamin D deficiency Is a disease in children where the growing bone becomes soft or rubbery are are easily deformed

Parathyroid hormone

Secreted by the parathyroid galnds Increases blood Ca2+ level Operates via a negative feedback system Also acts on the kidneys to decrease the loss of Ca2+ from urine Stimulates the formation of Vitamin D Promotes absorption of calcium from foods

Zone of Proliferating Cartilage

Slightly larger chondrocytes are arranged like stacks of coins Undergo interstitial growth as they divide and secrete extracellular matrix Divide to replace those that die at the diaphyseal side of the epiphyseal plate

Periosteal Arteries

Small arteries accompanied by nerves, enter the diaphysis through many perforating canals and supply the periosteum and outer part of the compact bone

Compact Bone Tissue

Strongest bone tissue Found beneath periosteum of all bones and makes up the bulk of the diaphysis of long bones Provides protection and support and resists the stresses produced by weight and movement

Open (Compound) Fracture

The broken ends of the bone protrude through the skin

Epiphyseal Arteries

The epiphyseal arteries enter the epiphyses of a long bone and supply the red bone marrow and bone tissue of the epiphyses.

Endochondral Ossification Formation of Articular Cartilage and the Epiphyseal (Growth) Plate

The hyaline cartilage that covers the epiphyses becomes the articular cartilage Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal growth plate The region responsible for the lengthwise growth of bone

Metaphyseal Arteries

The metaphyseal arteries enter the metaphyses of long bones and supply the red bone marrow and bone tissue of the metaphyses

Comminuted

The one is splintered, crushed, or broken into pieces at the site of the impact, and smaller bone fragments lie between the two main fragments

Ossification or Osteogenesis

The process by which bone is formed

Support

The skeletal system is the structural framework for the body by supporting soft tissues and providing attachments for tendons

Vitamins

Vitamin A stimulates activity of osteoblasts Vitamin C is needed for synthesis of collagen, the main bone protein Vitamin D helps with the absorption of calcium from foods in the gastrointestinal tract into the blood Vitamin K and B12 also needed for synthesis of bone proteins

Endochondral Ossification Development of the Secondary Ossification Centers

When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop Usually around the time of birth Ossification centers spongy bone remains in the interior of the epiphyses No medullary cavity is formed here Secondary ossification proceeds outward from the center of the epiphyses toward the outer surface of the bone

Blood Cell Production

Within certain bones, red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis Red bone marrow consists of: developing blood cells, adipocytes, fibroblasts and macrophages within a network of reticular fibers In a newborn, all bone marrow is red and involved in hemopoiesis With increasing age, much of the bone marrow changes from red to yellow

Calcitonin

Works to decrease blood Ca2+ levels Secreted from the thyroid Inhibits activity of osteoclasts, speeds blood Ca2+ uptake by bone and accelerates Ca2+ deposition on bones

Triglyceride Storage

Yellow bone marrow consists mainly of adipose cells, which store triglycerides A potential chemical energy reserve

The epiphyseal (growth) plate is a layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones

Zone of Resting Cartilage Zone of Proliferating Cartilage Zone of Hypertrophic Cartilage Zone of Calcified Cartilage